Gas Speak Colloquium 2011

Determination of Soil Restraint Properties and Calculation of Virtual Anchor Lengths in Buried Pipelines

Daniel TianMechanical Engineer

KBR

Introduction

• What is Pipe Soil Interaction?• What is Pipe-Soil Interaction?– Behavior of buried pipeline in the surrounding soil– Pipe and soil together form the engineered systemPipe and soil together form the engineered system

• Soil Restraint Propertiesp– Relationship between soil resistance and pipe movement

• Virtual Anchor Length– Pipe section moving relative to the soil near directional change

Why now?

• Risk based pipeline design approach (AS2885)

• Challenges facing pipeline designin Australia– Higher pressures– Larger diameters

Telfer Brooklyn-Lara Iona-Orford QueenslandTelferPipeline

Brooklyn LaraPipeline

Iona OrfordPipeline

QueenslandCSG Pipeline

Size (DN) 250 500 450 1050

Length (km) 443 65 101 Over 1000g ( )

MAOP (MPag) 10.2 10.2 15.3 10.2 / 15.3

WT (mm) 4.7 7.9 9.1 14.1 / 18.7

Material API 5L X60 API 5L X70 API 5L X70 API 5L X70Material API 5L X60 API 5L X70 API 5L X70 API 5L X70

Onshore Pipeline Uplift, Uzbekistan

Subsea Oil Pipeline Failure, Brazil, 2000

Purpose of Calculating Pipe-Soil Interaction

• Soil restraint input into Pipe Stress Analysis (AutoPipe or Caesar II)• Virtual anchor length to define model boundary• Virtual anchor length to define model boundary

– Above-Ground Facilities - Buried Pipeline Sectionsp

(soil restraint is the fundamental part)

Compressor Station Major Water CrossingCompressor Station Major Water Crossing

Determination of Soil Restraint Properties

Longitudinal TransverseActual

Vertical Upward Vertical Downward

Elastic-Plastic Soil Springp g

Determination of Soil Restraint Properties

Soil and Pipe Properties:• Unit Weight ()

Elastic-Plastic Soil Springs:• Soil Stiffness (K1)

• Undrained Shear Strength (Su)• Angle of Internal Friction ()• Pipe Diameter (D)

• Ultimate Soil Resistance (P1)

ALAPipe Diameter (D)• Depth of Cover (H)

ALA

AutoPipe

Peng

ALA Method for Calculating Pipe-Soil Interaction

• Widely recognised and applied

• Advanced soil modeller in Caesar II

• Based on laboratory and field i t l i ti tiexperimental investigations

• Formulas available for design factors• Formulas available for design factors

• Limitation:Limitation:– Under-estimate vertical upward

resistance (Appendix B)(Appendix B)

AutoPipe Method for Calculating Pipe-Soil Interaction

• Detailed in AutoPipe Technical Reference Manual

• Based on laboratory and field i l i i iexperimental investigations

Li it ti• Limitations:– Design factors need to be

determined from tables and charts

(Pipe-Soil Appendix)

Peng’s Method for Calculating Pipe-Soil Interaction

• First published in 1978

• Basic Soil Modeller in Caesar II

• Limitations:P li i ti ti– Preliminary estimation

– Based on theoretical soil mechanics

(Chapter 10)

KBR Pipe-Soil Interaction Calculation Sheet

• Each of the 3 established methods• Each of the 3 established methods has strengths and weaknesses.

• Compare results from 3 methods and choose which method is suitableand choose which method is suitable for a particular application

• Calculate input data for stress software package

Typical Soil Properties

Soil TypeSoftClay

NormalClay

StiffClay

LooseSand

MediumSand

DenseSandy y y

Dry Unit Weight(kg/m3)

1600 1800 2000 1600 1800 2000

UndrainedShear Strength

(kP )5 25 100 0 0 0

(kPa)Internal Friction

Angle (°)0 0 0 25 30 40

• To be used when soil data is not available for critical locations

Angle ( )

• Soft clay – worst design case for unknown soily g

DN1000, 1.2m Cover, Soft Clay Condition, Various Methods

Vertical Upward

DN1000, 1.2m Cover, Soft Clay Condition, Various Methods

Transverse

DN1000, 1.2m Cover, Soft Clay Condition, Various Methods

Longitudinal VerticalDownward

Calculation of Virtual Anchor Length in Buried Pipeline

• Distance from bend, tee or A/G-U/G transition to the point where pipe axial strain is completely suppressed by soil.

La

Virtual Anchor Length La = Co A [ε E + (0.5 - v) SHP] / Plong

Elastic Factor

Stress -Thermal

Expansion

Stress -Pressure

Elongation

LongitudinalSoil Resistance

Expansion Elongation

Virtual Anchor Lengths under Different Soil Conditions

DN1000, 1200mm Cover, 55°C DT, 10.2MPag DP

Applications

Anchor Block Requirementat Pig Trap

MajorW tWater

Crossing

Overbend atTop of Hill

Depth of Cover ChangeDepth of Cover Change

Pipeline Design Engineer’s Responsibility

• Know how to use the right engineering design toolsengineering design tools

• Do some research and understand pipe-soilunderstand pipe-soil interaction

• Make sound engineering• Make sound engineering judgement

Questions?

Disclaimer

These materials contain information of a general nature and are provided for discussion purposes only. They do not in any p p p y y yway represent engineering advice and KBR does not warrant the accuracy, completeness or currency of the information in these materials. Any person who uses or relies on these materials doesthese materials. Any person who uses or relies on these materials does so entirely at their own risk.